Molybdenum disulfide is a known lubricant additive. Unfortunately, it has certain known disadvantages which are associated with the fact that it is insoluble in lubricating oils. Therefore, oil-soluble molybdenum and sulfur containing compounds have been proposed and investigated as lubricant additives. For example, in U.S. Pat. No. 2,951,040 an oil soluble molybdic xanthate is disclosed as being useful in lubricating compositions. Apparently, the molybdic xanthate decomposes under conditions of use to form an oil insoluble solid molybdenum sulfide on the metal surfaces being lubricated.
U.S. Pat. No. 3,419,589 discloses the use of certain sulfurized molybdenum (IV) dialkyl dithiocarbamates as lubricant additives. These additives are described as being oil soluble or at least capable of being easily suspended in oils.
U.S. Pat. No. 3,840,463 discloses the use of certain metal dialkyl dithiocarbamates or dithiophosphates in combination with metal-free additives containing sulfur and phosphorous.
The foregoing patents are listed as representative of the very many known molybdenum and sulfur containing lubricant additives.
As is known in the art, some lubricant additives function as antiwear agents, some as anti-friction agents, and some as extreme pressure agents. Indeed, some additives may satisfy more than one of these functions. For example, metal dialkyl dithiophosphates represent a class of additives which are known to exhibit antioxidant and antiwear properties. The most commonly used additives in this class are the zinc dialkyldithiophosphates. These compounds provide excellent oxidation resistance and exhibit superior antiwear properties. Unfortunately, they do not have the most desirable lubricity. Therefore, lubricating compositions containing these zinc compounds also require the inclusion of antifriction agents. This leads to other problems in formulating effective lubricant compositions.
Additionally, extreme care must be exercised in combining various additives to assure both compatibility and effectiveness. For example, antifriction agents affect the metal surfaces differently than antiwear agents. If each type of additive is present in a lubricant composition, each may compete for the surface of the metal parts which are subject to lubrication. This can lead to a lubricant that is less effective than expected based on the properties of the individual additive components.
Thus, there remains a need for an improved lubricating compositions based on standard lubricating oils and containing other conventional lubricant additives.